A mass spectrometry (MS) system in general includes an ion source for ionizing components of a sample of interest, a mass analyzer for separating the ions based on their differing mass-to-charge ratios (or m/z ratios, or more simply “masses”), an ion detector for counting the separated ions, and electronics for processing output signals from the ion detector as needed to produce a user-interpretable mass spectrum. Typically, the mass spectrum is a series of peaks indicative of the relative abundances of detected ions as a function of their m/z ratios. The mass spectrum may be utilized to determine the molecular structures of components of the sample, thereby enabling the sample to be qualitatively and quantitatively characterized.
In certain “hyphenated” or “hybrid” systems, the sample supplied to the ion source may first be subjected to a form of analytical separation. For example, in a gas chromatography-mass spectrometry (GC-MS) system, the output of the GC column may be transferred into the ion source through appropriate GC-MS interface hardware. In a gas chromatograph, a sample to be analyzed is carried in a gas stream (mobile phase) through the GC column, which includes a stationary phase that causes separation of different components of the sample. The gas-sample mixture is then introduced into the ion source. For GC-MS, the ion source is typically an electron impact (EI) source, chemical ionization (CI) source, or a photo-ionization (PI) source.
The mass analyzer must be operated at a very low pressure (e.g., less than 10−5 Torr) to avoid ion-molecule reactions. Also, the ion source is typically operated at a very low pressure (e.g., less than 10−3 Torr) to facilitate interfacing with the mass analyzer and in some cases (e.g., EI) to ensure proper operation. Consequently, to preserve vacuum conditions the gas flow into the mass spectrometer is limited to a small rate, typically 1-10 mL/min. Therefore, in a typical GC-MS system the GC column has a very small bore (i.e., a capillary column), with an inside diameter typically not larger than 0.5 mm. In contrast to the mass spectrometer, the gas chromatograph typically operates at atmospheric pressure (about 760 Torr, or 1 atm) at the column outlet to achieve effective chromatographic separation. This large difference in operating pressures presents a challenge when coupling a gas chromatograph to a mass spectrometer. In some known GC-MS systems, ions produced in the ion source are transferred to the mass analyzer by way of a small-bore aperture. The low molecular weight carrier gas (e.g., helium, nitrogen, argon, or hydrogen) from the GC column is preferentially removed from ion source via a vacuum pump, while the heavier analyte ions are drawn into the sampling aperture. The sampling aperture is small enough to avoid breaking the vacuum inside the mass spectrometer. However, the small size of the sampling aperture results in low ion collection efficiency. That is, most ions produced in the ion source do not enter the sampling capillary, and thus are not mass-analyzed and do not contribute the ion signal utilized to construct the mass spectrum.
The use of a multi-bore capillary column or a multi-channel column (or multi-capillary column, or MCC) as a GC column is of interest. An MCC may consist of a bundle of hundreds of individual capillaries or a single tube with multiple channels, each capillary or channel providing a stationary phase and defining an individual flow path for a gas-sample mixture. The MCC can enable fast-speed, high-resolution analysis while offering high-capacity gas-sample flow without sacrificing column efficiency. The column flow in an MCC may be, for example, two to three orders of magnitude higher than the flow in a typical single-capillary column. Due to the parallel operation of multiple capillaries, the column length can be shortened. The high capacity and short length of the MCC can reduce GC analysis time down to a few minutes. The higher flow rate also enables isothermal separation of volatile organic compounds at ambient temperature.
The MCC, with its high capacity (high flow rate and high total gas flow), may be compatible for coupling with a traditional GC detector employed in a one-dimensional analysis (e.g., when a mass spectrometer is not utilized as the detector), such as a flame ionization detector (FID) or a thermal conductivity detector (TCD). Due to its inherent design (e.g., high capacity and operating pressure), however, the MCC is not readily adaptable for use in conjunction with a mass spectrometer. In a previous investigation, a heated jet separator was positioned outside an evacuated ionization chamber (10−6 mbar) of a mass spectrometer. The jet separator was coupled between an MCC (900 capillaries, total gas flow rate on the order of 200 mL/min) in a GC oven and an EI source in the ionization chamber. Inside the jet separator housing, an expansion chamber separated the end of the MCC outlet and the beginning of an aperture leading to the EI source. The expansion chamber was in open communication with a vacuum port leading to a rotary pump to pump carrier gas away from the higher-mass analyte molecules. Such a configuration does not provide an acceptable level of ion collection and transfer efficiency.
Therefore, there is a need for systems, devices and methods for interfacing an MCC with a mass spectrometer. For instance, it would be desirable to provide a solution for interfacing an MCC gas chromatograph with a mass spectrometer in which the operating pressure is transitioned from that of the gas chromatograph down to the vacuum level of the mass spectrometer, while providing high ion collection and transfer efficiency, and while preserving the advantages of the MCC such as fast analysis and high capacity.